1. Field of the Invention
The present invention relates to nanocomposites, and, more particularly to boron nitride nanotube-boron nitride (BN—BN) nanocomposites.
2. Description of Related Art
Aerospace and other industrial applications require high-temperature sensing, actuation, communication and structural elements that are able to withstand extreme environments. For example, the power generation industry requires materials designed to operate under high temperatures, pressures, and, in the cases of nuclear power plants, radiation environments. Another example is the development of materials for high friction environments, such as aircraft brake pads, which require high wear resistance to prolong the lifetime of the material, excellent thermal conductivity to dissipate the heat generated, and high thermal stability in oxidative environments. Sensors, actuators and structural materials able to withstand extreme temperatures, as well as high impact forces, such as with the Space Shuttle leading edges, are desirable for a variety of tailored applications. BN—BN nanocomposite materials are an appropriate choice to replace carbon based nanocomposites for applications when insulating materials are necessary. Furthermore, controlling the amount and type of conductive material nanocomposites, such as the BCN ceramic, can be prepared with tailored electrical conductivity.
State of the art structural materials for high wear/friction conditions include carbon-carbon composites that suffer from oxidation at the high temperatures generated during their operation, as well as asbestos containing liners inclusions that have been shown to be a health risk.
Conventional ferroelectric ceramic powders such as lead zirconate titanates (Perovskite PZT) have been used with a polymer matrix to create flexible piezoelectric polymer composites for sensing and actuation. Their heavy weight, brittleness and toxicity have limited their use in aerospace applications to very low loading levels and their effectiveness as flexible sensors and actuators has been disappointing. Their Curie temperature, where they lose their piezoelectric properties, is only about 200° C. In addition, anti-penetration/wear resistance and radiation shielding materials have been widely required for space exploration but are limited in usefulness due to the previous mentioned restraints. Thus there is a requirement to develop robust materials exhibiting adequately high hardness/toughness and high radiation shielding properties while providing sensing and actuation capabilities at temperatures over 500° C. Such materials have as yet not been developed.
Recently, a series of amorphous piezoelectric polyimides containing polar functional groups have been developed, through molecular design and computational chemistry, for potential use as sensors in high temperature applications. The piezoelectric response of these polyimides is, however, an order of magnitude smaller than that of poly(vinylidene fluoride) (PVDF). This is due to the fact that the dipoles in the polymer do not align along the applied electric field efficiently because of limited chain mobility within the imidized closed ring structure. To increase the piezoelectric response of these polymers, synthesis with various monomers, control of the poling process, and addition of carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs) into polymer have been reported [Kang et al, Nano, 1, 77 (2006); Park et al, Adv Mater, 20, 2074 (2008)].
A new method of preparing BN film or fiber using a boron-nitrogen containing polymeric precursor was developed [Rousseau et al., U.S. Pat. No. 6,774,074]. Rousseau teaches the synthesis of a boron-nitrogen containing polymer to make a green body according to Wagner's work [Wagner et al, Inorganic Chemistry, 1, 99, (1962)], then obtaining a BN ceramic fiber or film by pyrolysis of the green body. The BN ceramic obtained exhibited high modulus and hardness.
There are still limitations to the use of electroactive polyimide composites in many applications. For example, polymer based materials have the limitation of service temperatures well below 500° C. Also, the pyrolized BN materials taught by Wagner et al. exhibit excellent thermal stability but, like other ceramic materials, still possess very poor toughness. Carbon based, high hardness/toughness materials for high friction and structural applications suffer from degradation in oxidizing atmospheres.
It is a primary aim of the present invention to provide boron nitride nanotube-boron nitride (BN—BN) nanocomposites.
It is an object of the invention to provide BN—BN nanocomposites having enhanced toughness.
It is an object of the invention to provide BN—BN nanocomposites having enhanced hardness.
It is an object of the invention to provide BN—BN nanocomposites having enhanced radiation shielding properties.
It is an object of the invention to provide BN—BN nanocomposites having tailored photonic bandgap.
It is a further object of the invention to provide BN—BN nanocomposites having enhanced photoluminescence.
Finally, it is an object of the present invention to accomplish the foregoing objectives in a simple and cost effective manner.
The above and further objects, details and advantages of the invention will become apparent from the following detailed description, when read in conjunction with the accompanying drawings.